Both naturally occurring and synthetic proteins, peptides and nucleic acids have highly interesting properties as regards to their uses as active substances and materials, but are often only available in very small amounts. Since it is also often not possible to obtain them efficiently under economic conditions and in sufficient amounts in recombinant host systems, e.g. in bacteria, such as Escherichia coli, Bacillus subtilis, etc., commercial use cannot be realized. In order to be able to obtain more complex proteins and peptides or nucleic acids which are difficult to produce or cannot be produced at all in lower organisms, cells of higher organisms having an inherent complex protein and nucleic acid biosynthesis machinery are increasingly required as host cells. Transgenic animals, plants, mosses, algae, etc. have offered themselves as new recombinant hosts for some years now. Due to the availability of increasing numbers of well characterized biomolecules from molecular research, use of such hosts is gaining in importance for their production.
However, in host organisms the content of transgene-coded biomolecules does not always lie within the desired range. In particular the yields obtained in the host cells for the production of proteins or nucleic acids on an industrial scale are insufficient. On the one hand, this is due to the regulation of gene expression and, on the other hand, also caused by a degradation of the transgenic products by the host organism. In general, an increase in the expression and an increase in the amount of biomolecules stored in the organism is desirable for increasing the content of transgenic proteins, peptides and nucleic acids. This could raise the efficiency of the production of biomolecules in transgenic organisms and subsequently facilitate their recovery and purification.
In order to obtain a high content of transgene-coded biomolecules, it is necessary to make use of those regulation mechanisms resulting in an increase of expression and to avoid or eliminate those suppressing the production or degrading the products. The use of strong promoters is a general approach for increasing transcription and thus raising the amount of mRNA made. This is usually also accompanied by an increase in the amount of foreign protein formed.
In order to protect once formed mRNA from an increased turnover, which plays a role in gene silencing, all measures preventing detection of RNA as foreign are suited. Such measures are e.g. the prevention of double-stranded RNA formation, the adaptation of the GC content to that of the host cell and the use of repressor proteins for suppressing post-translational gene silencing (De Wilde, Plant Molecular Biology 545 (2000), 347-359). By adapting the codon usage to that of the host cell it is possible to achieve an increase in translation. The transgene-coded biomolecule content can also be raised by lowering the formation of an endogenous storage protein, as accomplished by Goossens et al., (FEBS Letters 456 (1999), 160-164) by means of the antisense technology. Another possibility of raising the foreign protein expression in transgenic organisms is the construction of fusion proteins between the target proteins and e.g. chaperonins or chaperonin binding domains.
However, it has only been possible thus far to raise the content of the desired transgenic molecules in organisms to some degree by these methods. In order to render the production of biomolecules in transgenic organisms more efficient, a mechanism is highly required which can be used as such or also in addition to said methods and in this connection raises the transgenically encoded biomolecule content significantly.
The present invention is thus based on the technical problem of providing a means by which an increase in the transgenic biomolecule content can be achieved in organisms, in particular in plants.